1. Field of the Invention
The present invention relates to a self-extinguishing resin composition comprising a thermoplastic resin and magnesium hydroxide which is not susceptible to silver-colored streaking during extrusion or injection molding.
2. Description Of The Prior Art
Recently, the demand for flame-proof materials useful in household electric products has been increased. Blending with antimony trioxide, halides, or vinyl chloride resins, or copolymerizing with vinyl chloride monomer and the like, have been known as methods for flame-proofing thermoplastic resins. However, a significant disadvantage is the attendant formation of toxic and corrosive gases upon combustion of such products.
In the past, self-extinguishing resin compositions have been prepared based upon polypropylene and/or polyethylene which contain more than 45 wt. % of aluminum hyroxide, magnesium hydroxide, or a complex of magnesium hydroxide and magnesium carbonate. Especially favorable are the self-extinguishing resin compositions which comprise 55 - 25 wt. % of a polyolefin, 45 - 75 wt. % of magnesium hydroxide and a metal salt of a fatty acid. Such compositions are extruded for molding in order to control the melt index to lower than a predetermined value. When magnesium hydroxide is combined with a polyolefin, the resultant product is flame-retardant, does not form a toxic gas upon combustion and displays no toxicity caused by the magnesium hydroxide additive. Accordingly, the composition is remarkably advantageous as a self-extinguishing resin composition. On the other hand, it is difficult to control the fluidity of such a composition in the molten state in a range suitable for the application of conventional molding methods such as injection molding or extrusion molding. Moreover, the appearance of the resulting molded product is undesirable, especially as regards the formation of a silver-colored streak.
There are several other basic principles well known in the art. It is common knowledge that the flame-retarding property of a thermoplastic resin is decreased by blending carbon black, a fine carbon powder. Also, when a large amount of an inorganic filler is blended with a thermoplastic resin, the flame-retarding property is increased. When magnesium hydroxide is blended, the flame-retarding property is remarkably high, but if ignited, the blended composition will burn. Consequently, there remains a need for a fire-retardant composition which is suitable for molding.